Not Applicable
The present invention is directed to a remote service unit for use in an OFDM/DMT digital communications system. More particularly, the present invention is directed to an improved transmitter architecture for use in a remote service unit of a multi-point. OFDM/DMT digital communications system.
Multi-point communications systems having a primary site that is coupled for communication with a plurality of secondary sites are known. One such communications system type is a cable telephony system. Cable telephony systems transmit and receive telephone call communications over the same cable transmission media as used to receive cable television signals and other cable services.
One cable telephony system currently deployed and in commercial use is the Cablespan 2300 system available from Tellabs, Inc. The Cablespan 2300 system uses a head end unit that includes a primary transmitter and primary receiver disposed at a primary site. The head end unit transmits and receives telephony data to and from a plurality of remote service units that are located at respective secondary sites. This communication scheme uses TDM QPSK modulation for the data communications and can accommodate approximately thirty phone calls within the 1.9 MHz bandwidth typically allocated for such communications.
As the number of cable telephony subscribers increases over time, the increased use will strain the limited bandwidth allocated to the cable telephony system. Generally stated, there are two potential solutions to this bandwidth allocation problem that may be used separately or in conjunction with one another. First, the bandwidth allocated to cable telephony communications may be increased. Second, the available bandwidth may be used more efficiently. It is often impractical to increase the bandwidth allocated to the cable telephony system given the competition between services for the total bandwidth assigned to the cable service provider. Therefore, it is preferable to use the allocated bandwidth in a more efficient manner. One way in which the allocated bandwidth may be used more efficiently is to use a modulation scheme that is capable of transmitting more information within a given bandwidth than the TDM QPSK modulation scheme presently employed.
The present inventors have recognized that OFDM/DMT modulation schemes may provide such an increase in transmitted information for a given bandwidth. U.S. Pat. No. 5,539,777, issued Jul. 23, 1996, purports to disclose a DMT modulation scheme for use in a communications system. The system principally focuses on applications in which a single secondary site includes a plurality of differing receiver and transmitter devices. The transmitters and receivers used at the secondary site of the system described therein, however, are quite complex and require a substantial amount of processing power. As such, the system disclosed in the ""777 patent does not readily or economically lend itself to multi-point communications systems in which there are a large number of secondary sites each having at least one receiver.
A multi-point communications system is set forth herein. The communications system includes a receiver and transmitter disposed at a primary site for communication with a plurality of remote service units disposed at respective secondary sites. The receiver of the primary site receives OFDM/DMT signals over a number of transmission bins. A transmitter at the remote service unit includes an improved transmitter architecture. The transmitter comprises a first circuit for converting a signal into a first serial digital data stream and a second circuit for generating a second serial digital data stream from the first serial digital data stream of the first means. The second serial digital data stream is a digital representation of an OFDM/DMT signal that is to be transmitted to the receiver of the primary site and is generated from the first serial digital data stream through a modulated direct digital synthesis thereof. A third circuit is used to convert the second serial digital data stream into an analog OFDM/DMT signal for transmission to the receiver of the primary site.
Preferably, the second circuit comprises an addressable sine table containing digital data corresponding to at least a portion of a sine wave. The sine table provides the digital data at an output thereof in response to address input signals. A control circuit, responsive to the first digital data stream, is used to generate the address input signals to the addressable sine table, multiply the digital data output from the addressable sine table by a predetermined amplitude factor, and accumulate the amplified digital data occurring over a single sample cycle to thereby generate individual data elements of the second digital data stream. The control circuit preferably comprises an address control circuit for providing a sequence of addresses to the input of the addressable sine table to generate a sequence of digital data values at the output of the addressable sine table. The sequence of digital data values output from the addressable sine table corresponds to sine waves having frequency and phase characteristics corresponding to the frequency and phase characteristics of the OFDM/DMT signal that is to be transmitted to the receiver of the primary site. The control circuit also preferably comprises an amplitude control circuit for multiplying each digital data value output from the addressable sine table by a corresponding amplitude factor to generate the amplified digital data. The amplitude factor is dependent on the digital data of the first digital data stream. A summing circuit digitally sums the amplified digital data over the single sample cycle to generate an OFDM/DMT digital data value for each sample cycle.